Answer: D) Complex.
Explanation: a complex sentence is a sentence formed by an independent clause (a phrase that has complete meaning by itself) and a dependent or subordinate clause (a phrase that has a subject and a predicate but doesn't have complete meaning by itself). The given sentence is an example of a complex sentence, with an independent clause "the Jebusites were conquered" and a subordinate clause "Because David's army entered the city unexpectedly."